As the world warms, temperate forests could become a source of carbon dioxide emission rather than a sink according to a new study in the Proceedings of the National Academy of Sciences (PNAS). Scientists found that two forest sites in the U.S. (Wisconsin and North Carolina) emitted long-stored carbon from their soils when confronted with temperatures 10-20 degrees Fahrenheit (5.5-11.1 degrees Celsius) higher than average.

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"We found that decades-old carbon in surface soils is released to the atmosphere faster when temperatures become warmer," explained lead author Francesca Hopkins with the UNiversity of California Irvine in a press release. "This suggests that soils could accelerate global warming through a vicious cycle in which man-made warming releases carbon from soils to the atmosphere, which, in turn, would warm the planet more."

The researchers discovered that topsoils (15 centimeters and up) could release between 1,750-4,700 kilograms of carbon dioxide per square meter at higher temperatures. Most importantly the team found that older carbon (at least ten year old) was vulnerable to warming.

"Our results indicate that large amounts of carbon [...] were vulnerable to increased decomposition losses with warming. The fact that we saw similar results at the two sites, despite differences in soil carbon stabilization therein, suggests that the pattern we observed may apply more broadly," the researchers write, adding that "the large amount of carbon in forest soils globally suggests that soil carbon could become a source of atmospheric CO2 under global warming."

Forests are vital carbon storehouses through sequestering the greenhouse gas and storing it long-term in their soils. However this study raises concerns that if temperatures rise high enough, long-buried carbon could create a feedback cycle that would be difficult, if not impossible, to stall.